Doxorubicin is a well-established and highly effective chemotherapy drug commonly used to treat multiple cancers such as lymphoma, leukemia, ovary, lung, and breast cancer, but its use is limited by cardiotoxicity. Cardiotoxicity can range from asymptomatic reduction in left ventricular ejection fraction to highly symptomatic heart failure (Class III to IV). Acute doxorubicin-induced cardiotoxicity (DIC) occurs in ~11% of patients and long-term cardiotoxic side effects, which can manifest up to 10 years after treatment, are observed in up to 36% of patients. Currently, we cannot predict which patients will develop cardiotoxicity. A major hurdle in filling the significant gaps in our knowledge about the mechanisms of DIC and how best to prevent it is the lack of good human models, due to the inaccessibility of patient-specific human cardiomyocyte samples, and the difficulty in isolating and maintaining human cardiomyocytes in vitro. Animal models are limited by significant functional disparities between animal and human cardiomyocytes. This hurdle has now been overcome by the recent advances in the generation of human induced pluripotent stem cells (iPSCs), in which a patient's somatic cells can be reprogrammed to pluripotency and maintained indefinitely in vitro. These iPSCs can then be efficiently differentiated into iPSC-derived cardiomyocytes (iPSC-CMs) and further studied in detail. In our preliminary studies, we have developed and validated a set of tools for assessing DIC in human iPSC-CMs. We have established that iPSC-CMs, derived from patients who have developed DIC, accurately recapitulate the susceptibility phenotype in vitro. Single nucleotide polymorphism (SNP) studies have identified several SNPs that are predicted to be highly associated with DIC (P=10-9 to 10-5). Hence in Aim 1, we will generate iPSC lines from 100 cancer patients treated with doxorubicin, 50 of whom experienced cardiotoxicity and 50 did not. In Aim 2, we will use the assays established in our pilot studies to assess the susceptibility to DIC and perform RNA-seq and eQTL mapping to discover novel SNPs. In Aim 3, we will introduce a very well established DIC-related SNP into five control iPSC lines using transcription activator-like effector nucleases (TALENs) and assess the effect on DIC susceptibility. Hence the overall aim of this R01 proposal is to use patient-specific iPSC-CMs to help elucidate the molecular mechanisms of DIC.